(a) Field of the Invention
The present invention relates generally to an improved mobile digital radio system. More particularly, it relates to an improved mobile digital radio system wherein a plurality of satellites and terrestrial repeaters transmit identical information on separate frequency bands to a mobile receiver that processes the frequency bands to produce a single high quality output signal.
(b) Description of Related Art
Practical power limitations of transmission sources and signal propagation impairments can substantially reduce the quality of digital radio frequency transmissions. Such signal degradation is particularly troublesome for mobile receiver applications because the nature and severity of the signal corruption changes unpredictably as the mobile receiver unit moves within the operating environment.
One method of improving the mobile reception of digital radio frequency transmissions uses signal source spatial diversity in conjunction with a signal source selection scheme. With this method, a plurality of spatially diverse signal sources simultaneously transmit identical information on different frequency bands. A mobile receiver unit simultaneously monitors the signal quality of all the frequency bands, and selects the highest quality signal as its output. Thus, if one signal source becomes seriously impaired or blocked as the mobile receiver moves through the operating environment, the receiver will compose its output signal using another unpaired signal source transmitting identical information.
Another method of improving the mobile reception of digital radio frequency transmissions uses signal source temporal diversity in conjunction with a signal correction scheme. With this method, a plurality of temporally diverse signal sources transmit identical information on different frequency bands. Typically, two frequency bands transmitting identical information are used wherein one frequency band transmits with a time delay of several seconds relative to the other frequency band. Alternatively, a signal and its delayed copy can be time division multiplexed onto a single frequency band. A mobile receiver ordinarily uses the delayed signal for its output, and stores in memory a moving window of data from the leading signal equal to the relative time delay. Thus, if the delayed signal source becomes seriously impaired or blocked as the mobile receiver moves through the operating environment, the receiver will recompose its output signal using leading signal data retrieved from memory.
There is a growing industry trend and consumer demand for improved quality in mobile data and audio services. For example, automotive digital radio, Satellite Digital Audio Services (S-DARS), and satellite broadcast data services are promising markets. Mobile digital audio presents the most difficult challenge, though, because its viability depends on superior service coverage and output signal fidelity. Music in, particular, is extremely challenging because the human ear can perceive even the smallest distortions in music as objectionable.
Existing mobile digital radio systems cannot produce the signal fidelity needed for high quality digital audio. Satellite based systems cannot achieve the needed spatial diversity of transmissions. Although satellite transmissions of digital radio signals can be economically dispersed over wide geographic areas, numerous coverage gaps are created in urban areas by trees, buildings, and other structures. As a result, mobile satellite users will frequently experience signal fading and loss as they travel through urban areas. Furthermore, increasing satellite power margin to penetrate buildings and to overcome common shawdowing effects would be cost prohibitive. Existing terrestrial digital radio systems are also inadequate. Terrestrial systems suffer from multi-path effects such as signal fading and signal cancellation, and they are cost prohibitive in rural areas where the population is sparse.
Consequently, a need exists for an improved mobile digital radio system. More specifically, there is a need for a system that can economically provide superior service coverage, and that can produce a high quality audio output.